Conventional techniques and apparatus employed in an electrophotographic machine for both making copies from an original document, and printing using data stored in a buffer, are known. Electrophotographic machines incorporating these techniques and features can have both a copy mode and a print mode.
In a typical copier/printer machine, an endless run of photoconductor medium having multiple page image areas carried by an electrophotographic drum is first subjected to an electrostatic station for uniform charging. An endless run of photoconductor in this context is defined to include a continually rotated photoconductor. The photoconductor may be subjected to occasional indexing around the drum from a supply roll to a take-up roll, both of which are internal to the drum. In the copy mode, a page image area on the photoconductor is then selectively imaged by light reflected from an original document. In the print mode, the photoconductor page area is imaged instead by a laser beam which is controlled by a text-defining binary data, for example, as described in U.S. Pat. No. 3,898,627. A character generator, as described in the reference patent, is enabled to drive the laser beam only after a page buffer is filled with a full page.
In either modes, the resulting electrostatic latent image is then toned by a developer by applying a toner to the latent image. The developed image is then removed from the photoconductor image area to a copy sheet for subsequent fixing, as the drum rotates in a given direction at a constant speed.
While the above described electrophotographic process operates synchronously with the rotation of the drum, the time it takes to assemble the text and to fill the page buffer to its full-page condition, as in the print mode, is page data content dependent. A full-page condition of the page buffer in this context is defined to include a page with a variable record length and a variable amount of blank space on the page. Hence, the readiness of the page buffer may be such that it does not permit synchronous operation with the associated electrophotographic process. Depending on the page data content, it is thus possible that one page image area can be used over and over while the remaining page image areas are unused for a number of consecutive print cycles. When this occurs, the unused page image area on the photoconductor medium experiences graduate "toner cloud" build-up. This toner cloud build-up is the result of an accumulation of excess or non-transferred charged toner particles between successive print cycles.
The phenomenon of toner cloud build-up, also known as "toner filming", tends to affect adversely the quality of the first prints which are made when any one of the unused page image areas is used to produce a page after a number of consecutive unused print cycles. More specifically, this undesirable effect shows up as excessively high background level on such first prints produced.
This toner filming problem is particularly noticeable if a page image area passes through unused for five or more consecutive print cycles. Such unbalanced page image area usage could happen while in the print mode because of the above described asynchronous operation of the page buffer with the associated electrophotographic process. The undesirable high background level is especially severe and unacceptable in electrophotographic machines using a particular type of toner composition, or in such machines wherein the electrophotographic process is designed to operate at a slow speed so as to optimize other print/copy qualities, such as optical density, offset master quality, and recycle rate, in order to obtain overall high quality prints and copies.
The above described print quality degradation on the first prints produced could be substantially alleviated by limiting toner filming build-up on the photoconductor medium by subjecting the page image areas thereon to brush cleaning during every print cycle, regardless of usage. Brush cleaning is accomplished usually by scrubbing the photoconductor medium with fibers. A cleaning operation of this type occurring in every print cycle minimizes toner cloud build-up and therefore alleviates the problem of high background level.
Brush cleaning in every print cycle, however, is not an acceptable solution to the above-described toner filming problem. The relatively high ambient temperature inside a typical electrophotographic machine causes the brush fibers, upon rubbing, to react with the photoconductor medium, whereby a thin layer of Teflon, also known as "clear filming" is deposited on the photoconductor medium. As a result, brush cleaning of the photoconductor medium in every print cycle produces a repeated accumulation of clear filming on the unused page image areas. This repeated accumulation causes an unacceptable "wash-out" effect, i.e., poor optical density on the first print produced when an unused page image area is employed to produce a print after not being used in several consecutive print cycles. Such a wash-out degradation is especially noticeable in half-tone printing.
Where the brush fibers are in physical contact with the photoconductor medium for a longer time period as is in the aforementioned electrophotographic process designed to operate at slow speed, the wash-out degradation in print quality is particularly severe and becomes even more intolerable.
The problem of toner filming has been encountered and noted in some earlier copier designs, however, none provides a solution to the problem in accordance to the invention described hereinafter.
Some prior electrophotographic copiers provide a scheme whereby forced alternate use of page image areas is achieved. As an example, U.S. Pat. No. Re. 29,179 to Davidge et al, discloses, in one embodiment, a reproduction apparatus having an electrophotographic plate containing thereon multiple imaging areas to effect alternate redevelopment and reimaging cycles when producing more than one copy of the same original document. This sequencing scheme results in situations where an image area can be skipped in order to facilitate maximum throughput.
Other prior electrophotographic copiers provide a scheme whereby an endless run of photoconductor medium is used uniformly for copying. An endless run of photoconductor in this context is defined to include a continually rotated photoconductor. The photoconductor may be subjected to occasional indexing around the drum from a supply roll to a take-up roll, both of which are internal to the drum. For instance, U.S. Pat. No. 4,139,300 to Katayama et al discloses a control system for a copier wherein clock pulses are generated per complete rotation of an endless photosensitive drum. The clock pulses are counted, and are utilized thereby to end each of the copier cycles by positioning the photoconductor medium at a rotational position different from its start position in that cycle so as to uniformly use the photoconductor, and elongate the life of the photoconductor medium.
Another prior electrophotographic copier providing means for more uniform photoconductor area usage is disclosed in U.S. Pat. No. 3,785,730 to Weber et al. The reference patent employs a control circuit having a marker and a marker detector. An elongated flexible electrophotographic belt is first marked in a run, and the mark is then detected during a following run. The position of the belt relative to a reproduction commencement position is varied in accordance with the mark such that the image-forming surfaces for the following run are positioned in areas between image-forming surfaces for the proceeding run to insure that different areas of the belt are used.